This invention relates in general to off-highway vehicles. It relates particularly to the heating, ventilating and air conditioning (HVAC) system in an off-highway vehicle and, more particularly, to a purge control for the fresh air filter in the HVAC system for an off-highway vehicle.
Off-highway vehicles are often operated in extremely dusty environments. As a result, the HVAC systems for off-highway vehicle cabs must employ highly effective fresh air filters to remove dust and other particulate matter before it enters the cab. They frequently use what is known as fresh air filter purging to dislodge dust and other accumulated debris from the fresh air filter medium. Purging the filter causes dirt to be released from the inlet face of the filter medium, thus prolonging the filter service life.
Typically, an off-highway vehicle cab is equipped with one of two types of interior air recirculation systems. One is known as a fixed system and the other as a variable system.
In a variable system, the relative amounts of fresh air taken in and recirculated air can be varied, i.e., the ratio between fresh air circulation and recirculation of cab air can be adjusted. A variable recirculation system, in a minimum recirculation/maximum fresh air setting, provides the highest cab pressurization and the best dehumidification. When at the maximum recirculation/minimum fresh air setting, filter life is generally extended and the system provides the greatest potential heating and cooling. Modern cab heating and air conditioning systems equipped with Automatic Temperature Control (ATC) are capable of taking advantage of these characteristics and can automatically optimize the ratio of fresh air to recirculated air within the cab, depending on system settings made by the operator, cab temperature at the time and the ambient conditions encountered.
In a fixed system, no adjustment of the recirculation opening takes place. One type of fixed system is equipped with at least two blowers, one or more blowers dedicated to pressurizing the cab with outside air and one or more for recirculating air within the cab. A second type of fixed system utilizes one or more blowers, with each blower(s) both pressurizing the cab and recirculating the air within the cab.
In any of these systems, fresh air is introduced into the cab through a fresh air filter. In an effort to extend filter life, it is conventional to reverse the air flow direction through the fresh air filter to dislodge the accumulated dust and debris from the filter medium. One method of filter purging takes advantage of the air pressure pulse created inside the cab when the cab door is closed. The pressure pulse momentarily reverses the air flow through the filter, causing dust and dirt to be released from the filter.
With existing methods, the effectiveness of the purge is often less than ideal. Furthermore, the operator can experience some undesirable effects. The cabin pressure pulsation associated with the purge can be substantial and may cause operator discomfort. The same pulse also makes closing the cab door more difficult.
It would be a distinct advantage to provide a variable air circulation system that overcomes the above-noted shortcomings of the prior art.